The invention relates to electric lamps and particularly to electric lamps enclosed in a reflector. More particularly the invention is concerned with a reflector lamp (PAR) with a ceramic metal halide lamp capsule with a reduced lamp capsule seal temperature.
Ceramic lamp envelopes with modern metal halide seals have developed a new class of metal halide lamps (Geven et. al. in U.S. Pat. No. 5,424,609 and by Carleton et. al. in J. Ill. Eng. Soc. P139-145, Winter 1996 (Proc. Of IESNA Annual Conference)). These lamps contain metal halide fill chemistries, and two electrodes. A high voltage pulse between the electrodes is used to ignite the lamp. Normal current and voltage is then applied through the electrodes to excite the enclosed gas and fill materials to a plasma state. Typical fills include rare earth halides with various other additives including thallium halide and calcium halide, in addition to an inert starting gas such as argon or xenon.
The ceramic arc tube is often jacketed in another envelope, called an outer jacket, to protect the inner arc tube from the air. Many of the lamp parts, especially niobium electrical in-leads, oxidize rapidly if exposed to air at the lamp operating temperatures, causing the lamp to fail. These outer jackets are usually thermally isolated from the arc tube by construction and contain a vacuum or are filled with a partial pressure of an inert gas and a better material, for example a zirconium and aluminum compound, to better oxygen and hydrogen.
Often the inner arc tube and outer jacket are mounted inside a parabolic reflector (PAR or PAR lamp) to gather and direct the generated light from the lamp in a useful beam pattern. This can be a flood or a spot beam for illumination of interior surfaces or building facades in exterior applications. Such lamps with halogen light sources are also commonly used for illuminating merchandise in stores and outside lighting in residential applications, for example security lighting. There is great interest in using the ceramic metal halide lamps in the applications cited since they are efficient and provide excellent color rendering. The true colors or merchandise are rendered almost as if they were displayed in sunlight.
Economies of scale dictate using the same reflector for the new ceramic metal halide lamps (HCI lamps) as were originally used for halogen lamps. This keeps manufacturing costs to a minimum. It is also allows the lamps to be used in existing the fixtures.
Unfortunately, life tests have shown that the HCI lamps mounted in existing lamp structures fail prematurely at about 1500-2000 hours, instead of the rated 10,000 hours. This is attributed to the rapid chemical attack by the fill material on the sealing glass (frit) used to make the conventional HCI seals (see Geven et. al. in U.S. Pat. No. 5,424,609). The problem is exacerbated when the lamps are run in the base up configuration (base towards ceiling), as they are in many interior down lighting applications. The seal is then subject to greater heat and therefore more active chemical reaction. To be a useful product in the markets mentioned, the lifetime of the lamp must be extended.
A PAR lamp with an HID light source may achieve improved life by including a light absorbing layer in the neck of the reflector. An HID light source having two sealed electrodes defining a lamp axis is preferred. A concave ceramic shell is formed having an internal surface with a reflective surface. The shell further has a neck defining a neck cavity and a reflector axis. The neck is provided with an electrical connection and a mechanical support for the light source. The shell is positioned to surround the source and thereby reflect light from the source to a field to be illuminated during lamp operation. The light source and reflector are oriented with the lamp axis to be substantially co-axial with the reflector axis, with at least a portion of at least one of the electrodes extending in the neck cavity. A substantially non-transmissive, light absorbing layer that intercepts light from the source emitted in the direction of the neck is positioned in the neck to absorb light that might otherwise be reflected back onto the lamp seal region.